A brushless DC motor of the present invention is a brushless DC motor comprising a rotor with an attached permanent magnet, and a stator. The rotor with attached permanent magnet is assembled on a shaft and inserted in the stator, and the rotor and shaft are operable to spin within the stator on one or more bearings, when electrically excited.
Typical applications for a brushless DC motor on a powertrain for a vehicle may include, by way of example, a smart remote actuator for a cruise control system, an actuator and control device for a variable geometry turbocharger, or an exhaust gas recirculation valve actuator.
There are many schemes for controlling a brushless DC motor, depending upon the application of the motor, complexity of the motor, and types of feedback control devices used by the motor, such as position monitoring sensors and current monitoring sensors. Each control scheme employs some form of motor drive circuit comprising a plurality of switches, e.g. transistors, operable to create successive commutation states (i.e., wherein each motor phase is either HIGH, LOW, or, OFF) in electrical windings of the stator, thus inducing successive magnetic fields that cause the rotor with attached permanent magnet to rotate on the shaft. Two typical control schemes for a brushless DC motor include a peak torque commutation method, and, a zero torque commutation method. The peak torque commutation method describes a control system wherein the commutation state is selected so the motor generates maximum possible torque at a desired position. The peak torque commutation method typically comprises a closed loop control algorithm wherein magnitude of electrical current supplied to the brushless DC motor is controlled so motor output torque is balanced against an external mechanical load applied to the brushless DC motor via a motor shaft. A typical peak torque commutation method has a disadvantage when the external mechanical load is relatively low relative to motor effort capability, or output torque capability, of the electric motor. In such situations, the corresponding motor effort, or torque, is low, and therefore a minimal force is able to deflect the motor shaft away from the commanded, or desired, position. This ability to readily deflect the motor shaft away from the commanded position may affect performance of the output device being controlled, and may cause other deleterious effects in the system, such as uncontrolled resonance in the output device. Furthermore, the controller may be unable to maintain the output device at the commanded position if there are sudden changes in the amount of external mechanical load.
The zero torque commutation method describes an open-loop control system wherein the aforementioned commutation state is selected so desired rotational position of the brushless DC motor and motor shaft correlates to the point at which the selected commutation state torque curve is at zero torque. In operation, the motor is able to respond with appropriate opposing force when an external mechanical load is applied to the motor shaft and motor, thus reducing or essentially preventing deflection of the motor shaft. The brushless DC motor generates opposing torque against any load fluctuation, without position-feedback control. A typical zero torque commutation method has a disadvantage that the brushless DC motor operates at less than peak energy conversion efficiency (torque v. motor current).
Therefore, it is desirable to employ a control scheme for a brushless DC motor that is able to selectively use either the open-loop zero torque commutation method or the closed-loop peak torque commutation method, depending upon the specific motor operating conditions, to reduce the ability of an external force to deflect the brushless DC motor and motor shaft from controlled position, while optimizing energy conversion efficiency of the brushless DC motor.